Most Windows PCs sold today come with a hidden partition containing a restore function. This partition will often have no drive letter assigned and will not be visible in Windows Explorer. It may be possible to make the partition accessible by using Diskpart to assign a drive letter to the partition. However, there are limitations and according to Mivrosoft not all hidden partitions can be assigned this way. Here is the procedure. Enter the following sequence of commands:

diskpartlist diskselect disk 0 (zero is the normal numbering for the restore partition but be sure to check if this is correct by looking at the sizes)list partitionselect partition 1 (or the smaller partition with no drive letter if the numbering is different)assign X (X is whatever drive letter you want to use)exit